Energy band mixing induced in molecular systems involving electron rich and poor π-conjugated substituents, a “donor-acceptor” (DA) approach, has found application in the design and synthesis of: light-emitting semiconductors; n-channel and ambipolar semiconductors for organic thin-film transistors; active organic components for chemical biosensors; and non-emissive organic electrochromics. Another significant application is directed toward low-bandgap photovoltaics that absorb in the visible and/or near infra-red (NIR) regions of the electromagnetic spectrum, particularly for bulk-heterojunction solar cells with improved solar energy conversion. Donor-acceptor π-conjugated polymers are attractive for use in innovative high-performance flexible light-harvesting technologies. These DA π-conjugated polymers allow easy bandgap engineering via structural control and allow mechanical deformability that can enable flexible electronic systems. Additionally, DA π-conjugated polymers have the potential for fabrication in a manner that is scalable at a low-cost and permits a high-throughput manufacture of light-harvesting devices. The DA π-conjugated polymer solution has considerable advantages over approaches employing inorganic equivalents for applications such as large-area solar cells that can be used for vehicle or housing roofs or for portable electronic devices made with finely printed photoactive arrays.
Although the donor-acceptor theory was first reported for macromolecular systems by Having a et al., Synth. Met. 1993, 55, 299, it has only recently been demonstrated with synthetic conducting polymers where good performance has been achieved in the context of photovoltaic cell efficiency. Efficiencies of up to about 5% have been demonstrated in systems, but the norm for practical systems has been significantly less. In general, the low power conversion efficiencies obtained using DA π-conjugated polymers can be attributed to: 1) their absorption spectrum is often limited to a small portion of the visible spectrum (typically the red region); and 2) their intrinsic charge carrier mobilities are low when used in solid state devices.
Two-band absorbing materials reflecting the color green are those that display two absorbance maxima in the visible light region with a window of transmission in the 480-560 nm range. In spite of about two decades of intense research on photovoltaic polymers, few two-band absorbing green polymeric photovoltaics have been reported, primarily due to the complexity of engineering the molecular structure of the repeat units that form the conjugated polymeric material. Simultaneous achievement of the required optical properties and desired charge-carrier mobilities has not been reported. To date, reported charge carrier mobilities have been low for such polymeric materials, which limit charge transport in solid state devices and has not enabled viable devices for solar energy conversion.
The neutral state green π-conjugated polymers, two-band absorbing materials, described in the literature to date, have disappointing charge-carrier mobilities, which in turn have not encouraged their use in light harvesting devices. The disappointing charge-carrier mobility of these polymers is due to: 1) a lack of favorable intermolecular interactions, primarily poor π-stacking; 2) an unfavourably large chain-to-chain distances (or lamellar spacing); and 3) relatively little extended conjugation due to a low level of planarity of the polymer's main-chains. Effective bulk-heterojunction solar cells require high charge carrier mobilities so that photo-generated excitons (geminate electron-hole pairs) can undergo diffusion and dissociation processes within the active layer of the device with subsequent rapid transport of the dissociated charges to collection electrodes. Unless a solar cell material displays sufficiently high charge carrier mobility, dissociated charges recombine before collection and the device exhibits poor solar energy conversion. Hence the development of a green π-conjugated polymer with high charge carrier mobilities that is readily processable is desirable for light harvesting applications.